1. Field of the Invention
The present invention relates to a semiconductor device using silicon carbide, and more particularly, to a silicon carbide semiconductor device with a high-reliability front surface electrode structure which covers the entire Schottky contact metal of an edge portion with front surface electrode metal to prevent the generation of an etching residue.
2. Description of the Related Art
In the related art, semiconductor devices using silicon (Si) as a semiconductor material have been mainly used as power devices. However, silicon carbide (SiC) which is a wide band gap semiconductor has the following physical property values: thermal conductivity that is three times more than that of silicon; maximum electric field intensity that is ten times more than that of silicon; and an electron drift speed that is two times more than that of silicon. Therefore, in recent years, each institute has conducted research on the application of power devices which have a high breakdown voltage and low loss and can operate at a high temperature.
In a structure of such a power device, a vertical semiconductor device has been mainly used in which a rear surface electrode including a low-resistance ohmic electrode is provided on the rear surface side of a substrate. The rear surface electrode is made of various materials and has various structures. As an example of the structures, for example, a laminate of a titanium (Ti) layer, a nickel (Ni) layer, and a silver (Ag) layer, for example, see JP 2007-184571 A (Patent Document 1), or a laminate of a titanium layer, a nickel layer, and a gold (Au) layer, for example, see JP 2010-86999 A (Patent Document 2), has been proposed.
In a vertical semiconductor device using SiC which is typified by a Schottky barrier diode, the following device has been proposed. For example, an n-type drift layer with low impurity concentration is deposited on one main surface of a SiC substrate having a (0001) plane as the main surface. The n-type drift layer includes a region A with a structure in which only a first conductivity type region is provided or a structure in which the first conductivity type region and a second conductivity type region are periodically provided, a region B of the second conductivity type which surrounds the region A, and a region C of the second conductivity type which surrounds the region B and has a different impurity concentration from the region B. In the front surface electrode, a metal layer which is made of aluminum (Al) or aluminum alloy is formed on a Schottky contact to form an electrode structure.
In addition, a method has been used which forms a nickel layer on the other main surface of a SiC substrate, heats the nickel layer to form a nickel silicide layer, and forms an ohmic contact between the SiC substrate and the nickel silicide layer, for example, see JP 2007-184571 A (Patent Document 1) and JP 2010-86999 A (Patent Document 2). Furthermore, as a method for forming an ohmic electrode, a method has been proposed which forms a plurality of metal films on a SiC substrate and then performs a heating process at a temperature of 700° C. to 1100° C., more preferably, about 800° C. to obtain good ohmic characteristics, for example, see JP 2005-277240 A (Patent Document 3). Further, JP 2008-135611 A (Patent Document 4) discloses a technique which radiates a laser beam to form an ohmic electrode.
In a process for manufacturing a Schottky barrier diode according to the related art, the front surface electrode is formed by a so-called lift-off method which forms a metal film on a resist having a window portion formed therein and then removes the resist, for example, see JP 2010-165838 A (Patent Document 5). However, in the lift-off method, a burr is likely to be generated in a pattern edge portion, which causes an element defect. Therefore, the lift-off method is not preferable. It is preferable to pattern the metal layer using a dry etching method, instead of the lift-off method.
When the metal layer is patterned by the dry etching method, a residue which is generated during dry etching is attached to a portion of the metal layer and the Schottky contact which is not covered with the metal layer is exposed to plasma and is damaged, which is likely to cause an element defect.
The invention has been made in order to solve the problems of the related art and an object of the invention is to provide a silicon carbide semiconductor device which does not cause an element defect and has high quality.